September will see one planet come to the fore while two others fade away. Both Mercury and Mars will drop out of view later this month, but not before they have a fascinating conjunction with brilliant Venus. The sky's brightest point of light serves as a beacon to find the fainter planets in evening twilight.

The action begins soon after sunset. Venus appears first in the deepening twilight. You should be able to spot the planet with your naked eye low in the west 30 minutes after sunset. Venus shines some 30 times brighter than Mercury, and Mercury appears five times brighter than Mars. Binoculars will yield the best views of the two fainter planets. All three tango against the backdrop of the constellation Virgo the Maiden.

If you watch the planets this week, you'll see Venus and Mercury move in step as they close in on Mars. On September 8, Venus lies a bit to Mars's right, with Mercury positioned to the lower left of the pair. By the 10th, Venus sits barely one Moon-diameter to the right of Mars. One evening later, the two lie half as far apart, with Venus standing above Mars. Mercury remains to the pair's lower left all week. On September 18, look for Venus to the upper right of Virgo's brightest star, Spica.

The three planets have different fates. Mercury's path carries it into the Sun's glare during the week of September 21. Because Mars follows a more leisurely orbit, it manages to stick around until early October. Its faintness and low altitude, however, will make it hard to see. Venus, on the other hand, has just begun a new season of visibility. Although it will stay near the horizon for the next couple of months, it will climb high in the southwestern evening sky by December.

Illustration: Roen Kelly

From the editors of "Astronomy" - the world's most popular astronomy magazine.

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